The invention relates in particular to a cryostat system that can be kept at a cryogenic operating temperature in a dry manner, i.e. without providing or supplying cryogenic fluids, with a cryocooler. In addition to the cryocooler, the cryostat system includes a superconducting magnet arrangement and a heat sink apparatus. The heat sink apparatus prolongs the time before the superconducting magnet arrangement returns to the normally conducting state (i.e., “time to quench”) if the active cooling of the cryocooler fails. A similar cryostat system of this kind, which does however still contain small amounts of a liquid cryogen, is known, e.g. from EP 0 937 953 A1 or DE 199 14 778 B4.
Superconducting magnets are cooled to cryogenic temperatures in order to function. Many users prefer “cryogen-free” magnets for this purpose, which forgo the use of cryogens (e.g. liquid helium and/or liquid nitrogen) entirely and maintain the operating temperature exclusively through use of “cryocoolers”, i.e. in a “dry” manner. Pulse tube, Stirling or Gifford-McMahon coolers are typically used in this context.
A cryogen-free magnet system of this kind typically has a very short time to quench the superconducting magnet system. If the cryocooler malfunctions (e.g., as a result of a power outage, an interruption to the cooling water supply, or a mechanical defect in the compressor or cold head), the superconducting magnet system very quickly heats up beyond the allowable operating temperature and quenches. The magnet system then cannot be used for a long time, since it must be cooled back down and recharged.
One possibility for increasing the time to quench consists in providing small amounts of a cryogen to evaporate when the active cooling of the cryocooler fails. The evaporation heat of the cryogen keeps the temperature of the magnet constant in the event of a cooler malfunction. This technology is commonly referred to as “Minimum Condensed Volume.” This technology is disadvantageous in that, strictly speaking, even the use of small amounts of a cryogen is not compatible with the definition of “cryogen-free”. Moreover, a system must be provided in order to condense the cryogen during the cooling of the magnet. Another disadvantage is the limited number of practical cryogens that can be used, resulting in only a few operating temperature ranges (e.g. approximately 2.5K-4.5K for He, approximately 70K-77K for LN2, etc.).
In the European patent reference EP 0 937 953 A1, the time to quench is prolonged by storing small amounts of a cryogen in a reservoir provided for this purpose. The coil itself is located within this reservoir. Although said reservoir is designed so as to be significantly smaller than a “conventional” helium tank in order to keep the amount of cryogen as small as possible, this arrangement is not truly “dry” in the strict sense of the term.
In the cryostat arrangement according to German patent reference DE 199 14 778 B4, the time to quench is also prolonged by storing small amounts of a cryogen in a reservoir provided for this purpose. However, the coil itself is located in the vacuum and is thermally connected to the reservoir.
In the cryostat system described in the German patent reference DE 10 2014 218 773 A1, the objective is also that of prolonging the time to quench. However, a completely different approach is taken to prolong the time to quench. In this reference, the thermal coupling between the coil and the cooler is reduced if the active cooler fails, minimizing the heat path to the superconducting magnet through the failed cooler.